The present invention relates to a radio base station apparatus having a plurality of antennas, and a radio communication method.
A conventional radio communication apparatus is explained below, based on a reference, xe2x80x9cTransmission Characteristic of W-CDMA/TDD System for Next Generation,xe2x80x9d IEICE Technical Report, SSE97-41, RCS97-36, The Institute of Electronics, Information, and Communication Engineers, Japan, June 1997. This reference discloses transmission antenna change diversity in a TDD (Time Division Duplex) system, as a type of space diversity, as follows.
A base station (BS) is provided with a plurality of antenna branches, combines diversity receptions in an up link, and employs transmission selection space diversity (SD) in a down link. In the up link, correlation values of respective branches and respective paths are combined in accordance with the maximal-ratio combining to perform demodulation. In the down link, received signal powers of communication channels are obtained by averaging the correlation values in the respective branches in the up link for a preceding slot (0.625 msec), and one of the antennas which maximizes received signal power is selected to transmit a slot (0.625 msec) in the down link through the selected antenna.
That is, according to the conventional technique, the correlation values (which correspond to received signal powers of desired signals) are calculated for the preceding slot for the respective branches (i.e., for the respective antennas) in the down link. Then, the antenna realizing a great received signal power is selected for transmitting a slot in the down link.
However, the above conventional technique has a problem. That is, since the same frequency is used in both the up and down links in the TDD system, the transmission antenna can be determined based on the received signals as above. On the other hand, different frequencies are used in transmission and reception in the frequency division duplex (FDD) systems. Therefore, a transmission antenna cannot be determined based on received signals as above for the transmission antenna change diversity in the FDD systems.
The object of the present invention is to provide a radio base station apparatus and a radio communication method whereby the radio base station can determine a transmission antenna based on received signals in frequency division duplex (FDD) communication.